Pests and Diseases 
Algal leaf spot (American Samoa) - Photos and report submitted by Dr. Fred Brooks, Plant Pathologist
The leaf spot lesions are a light brown color at first, usually surrounded by a
noticeable and diffuse yellow halo. In wet weather they are water-soaked; the centers
usually fall out of mature lesions.
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Close-up of algal leaf spot, showing brown colored lesion surrounded by a diffuse, yellow halo.
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Close-up image of the algal thallus on the surface of an infected noni leaf.
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Pathogen: The disease is caused by a plant-parasitic algae, Cephaleuros minimus. Cephaleuros species are the only plant parasitic algae known.
Symptoms: Leaf spots (approx. 1-2 cm diameter), characterized by a light brown color and surrounded by a conspicuous, diffuse, yellow halos. The pathogen, Cephaleuros minimus, enters the leaf though the upper surface and forms a filamentous thallus above the epidermis which moves intercellularly to the lower leaf surface. Sporangia and setae break through the lower epidermis and can usually be seen with a hand lens due to their bright orange color. The effect of these symptoms may be minor; the disase is not fatal but some premature defoliation may occur.
Disease distribution: As of 2005, the disease has only been reported from American Samoa. It is reported to be the most common leaf spot disease of noni in American Samoa.
Epidemiology: Algal leaf spot is favored by very wet, warm and humid weather and is most common where noni is growing in low-light conditions as an understory plant that is shaded by other species. Spores are dispersed by primarily splashing rain and by wind and wind-driven rain. |
| Control: |
- Sanitation (removal of severely diseased leaves from the
plant; removal and destruction of fallen infected leaves)
- Moisture and humidity management (ensure good drainage,
control weeds, adequate plant spacing, pruning, minimize
leaf wetness and overhead irrigation)
- Grow noni in full sun.
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| Notes: This is probably the first report of this disease in the Pacific and worldwide, although it is unlikely that the disease is actually confined to American Samoa. |
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